As a technique to separate a solid substance contained in a solution or a suspension from liquid, filtration with a filter has been conventionally known. As the filter used for the filtration, filters whose pore diameters differ depending on their intended use have been known. As a filter capable of separating and capturing a minute substance whose average particle diameter is 500 nm or less, those of resin have been conventionally in general use. However, it is difficult to improve capturing efficiency of a minute substance being a substance to be captured to a certain level or more.
Further, it is known that porous glass having a nano-order pore diameter is usable as a gas separation membrane (JP-A 2010-195612). To obtain this porous glass, glass is phase-separated and thereafter parts of its components are eluted by acid treatment. This Reference, however, gives no description regarding the separation of a minute substance contained in a solution or a suspension.